This invention relates to sensors and systems for measuring the concentration of oxygen present in a mixture of gases, wherein the sensor relies upon detection of thermal effects due to a wind formed in a magnetic field within the measuring device as a result of the paramagnetism of oxygen present in the mixture. A variety of such devices have been described for measuring oxygen concentration, and these magnetic wind oxygen sensing systems rely on the fact that when placed in a magnetic field, the paramagnetic oxygen will exert a pressure in a certain direction, while the remaining components of the mixture, which is substantially formed of diamagnetic gases, are unaffected by the local field. A mixture of oxygen with diamagnetic gases behaves, from the magnetic point of view, as a single component with a magnetic property (magnetic susceptibility) equal to the weighted average of the susceptibilities of each of the components.
One type of apparatus of this sort measures the concentration of oxygen by relying upon the inverse relationship between temperature and the magnetic susceptibility of oxygen, and provides a heater to raise the temperature of a portion of an oxygen-containing mixture in a local region of a magnetic field, producing a pressure differential that gives rise to a directional wind. The wind has a magnitude that depends on the local field, the local temperature differential or thermal gradient, and the level of oxygen. By arranging heating and heat sensing elements in close proximity so that movement of the wind carries heat or selectively cools one or another of the components, the magnitude of this wind, or its functional relationship to the oxygen concentration, may be calibrated.
A number of prior publications and patents describing instruments which exploit one or more of these effects are referenced in applicant""s earlier U.S. Pat. No. 5,269,170 issued Dec. 14, 1993; U.S. Pat. No. 5,012,669 issued May 7, 1991; and U.S. Pat. No. 4,893,495 issued Jan. 16, 1990. These patents are hereby incorporated herein by reference in their entirety, and attention is particularly directed to their circuit diagrams illustrating sensing bridge and temperature or current control arrangements for balancing the bridge. Among the complicating factors which must be corrected are the problem of defining a layout or geometry such that the so-called chimney effect, the natural directional flow induced by the lesser density of heated gas, does not confound the response of the thermal elements, and the problem of compensating for the rate of cooling or heating due to specific heat or heat capacity of the background gases present in addition to the measurand. In the foregoing patents a number of constructions are proposed for addressing these factors. On a circuit level these may include the use of multiple heating or sensing elements arranged in bridges to balance or counterbalance certain effects that enjoy symmetry as a result of their spatial layout. Another useful technique involves electrically heating a portion of the assembly to a constant temperature and monitoring the current required to maintain that temperature. This current may then be used to develop a normalizing measurement to which other parameters are referenced.
However, one basic limitation of this technology resides in the fact that small heated sensing elements are employed to detect the wind. The temperature of these elements is affected not only by the magnitude of the magnetic wind induced by the heating and magnetic field structures, but also by the heat transfer characteristics of the background gases that are present. A change in background gases thus induces a shift in zero point (i.e., the output when oxygen concentration is zero) of the sensing circuitry.
Furthermore, while constructions as illustrated in the aforesaid patents have enhanced the accuracy of paramagnetic oxygen sensing systems, they rely on the use of multiple elements in bridge configurations. This typically requires that the response and characterizing parameters of the elements be quite similar, i.e., that the components be matched. Some matching of the circuit characteristics of components is generally feasible, and may initially be performed quite accurately, especially for certain thin film devices wherein hundreds or thousands of virtually identical units are fabricated in a single process on a single wafer. However, initial matching of the basic response characteristics may be insufficient to assure continued accuracy. As a practical matter, when discrete sensors such as thermistors or resistive heating elements are used, the very process of mounting and arranging their geometry within the sensing device may introduce asymmetries of response, or instabilities of location that result in asymmetries of response over time. For example, when a thermistor is mounted close to the wall of a massive magnet structure, the rate of cooling due to gas conduction between the thermistor and the wall will vary with the composition of the background gas and its thermal capacity. Further, for a given background gas, such conductive dissipation is markedly affected by even small changes in proximity to the wall, which may introduce disproportionately large conductive or radiative heat loss, or with an opposite effect, may give rise to boundary layer flow stagnation.
Various approaches have been presented in the prior art to address the dependence on background gas thermal characteristics. For example, the above-cited patents teach a method and circuitry for maintaining the bridge at a constant temperature, and carrying out adjustments to compensate for background gas effects. However, such bridge circuitry may augment the variations induced by background gas changes, and the correction circuitry may not fully correct for these background-induced variations. Moreover, facially identical components in a bridge may respond differently to the same drive current when placed in series, because their thermal dissipation characteristics are not well matched. Mismatch may occur either intrinsically in the response of the circuit elements, or because one unit of a pair is positioned fifty or a hundred micrometers differently with respect to nearby structures. The small heated elements are also inherently subject to thermal stresses and temperature cycling, causing wires to shift and local geometry to change over time, introducing asymmetric effects, such as those just described, even in sets of initially well-positioned and well-matched components.
These effects can imbalance or impair the practical effectiveness of a bridge circuit, and may result in loss of calibration.
The problem is compounded because, since extremely minute levels of force are engendered by the action of a paramagnetic gas within the magnetic field, it is necessary that the heat sensing and generating elements be sufficiently small to make the effect of the induced wind detectable. It is further desirable that the sensors be mounted in sufficiently small passages that high flux may be achieved and also that wind is effectively channeled to develop higher velocity. However, because the small thermal elements necessarily are mounted on small conductors, normal flexing, structural bending, vibration and thermal expansion effects result in migration or shifting of the actual position of the heating and sensing element. Thus their response to wind-induced thermal transfer, or the rate at which each dissipates heat, or the power required to maintain a constant temperature, resistance or signal in the element, will vary over time as well as changing with properties of the background gases. This is particularly true of constructions in which the elements are both heated to generate a wind, and also employed as sensing elements to respond to heat transferred to or from the heated element.
One approach to this problem might be to provide a strictly planar device incorporating otherwise conventional sensing bridge circuitry, i.e., to provide a small chip having resistive heating and temperature sensing elements fabricated in a very precise array on the surface of the chip. However, such a construction may introduce problems of its own. Not only may the required metallizations for an intended sensing environment be incompatible with the metallochemistry of an otherwise desirable chip technology, but the use of strictly planar devices may be ill suited to a measurement apparatus that relies on small wind effects. This is because boundary layer effects, which greatly influence the gas flow being measured near to the surface of a planar device, may be difficult to model, or have asymptotic pole or null behavior, rendering the usual physical models inapplicable or subject to variations that would prevent calibration of the response with oxygen level.
Accordingly it would be desirable to provide a paramagnetic oxygen sensing apparatus having improved stability and predictability.
It would farther be desirable to provide a magnetic wind oxygen sensing apparatus that is less prone to variation and disturbance with changing composition of background gas in which the oxygen component appears.
One or more of the above desirable ends are achieved in accordance with the present invention by a magnetic wind oxygen sensing device that provides a local magnetic field is defined, for example, by magnetic pole pieces, and has a plurality of thermal elements arranged in a bridge at the local magnetic field to measure oxygen concentration present in a surrounding gas mixture. The device creates a magnetic wind and detects thermal effects induced in the elements as a result of the wind. The wind is generated by heating elements positioned at a region of high field intensity. The heat locally reduces paramagnetism of oxygen present in the gas, causing a pressure drop compared to unheated gas of identical composition, so that cooler gas in the magnetic field displaces the warmed gas and flows over the heater elements. Each wind generator is located between two sensing elements, and these are positioned so that, while both receive heat from the wind generator, one sensor is somewhat cooled (by flowing gas from the lower temperature region of the measuring cell as a whole, sometimes simply referred to as xe2x80x9cambientxe2x80x9d herein) and the other is somewhat heated, respectively, by the wind thereby generated. Significantly, in the absence of oxygen, there is no airflow, and the two sensing elements are at the same temperature (which is also the temperature of the gas immediately surrounding them), due to the heat received from the wind generator located centrally between them. As a result, the zero point remains fixed, even when conductive properties of the surrounding (but stationary) gas change. When oxygen is present, a wind arises, and the wind reduces the heat received by the upstream sensor and increases, by practically the same amount, the heat received by the downstream sensor. However, the total resistance of the two sensors remains substantially unchanged; this quantity can therefore be used to accurately control the level of the central, wind generating heater element.
Preferably the sensing elements are arranged in a bridge, and sensing is performed by detecting the resistance of the sensing elements with the elements attached to a very low current circuit, so that power dissipation in the element is negligible. The wind generator or heater elements may be powered at a variable level, responding to a feedback signal to maintain the temperature of sensing elements, or an output of a bridge circuit composed of sensing elements, at a constant level. The sensing portion may further employ a power supply that is completely independent from that of the wind generating heater elements, so that readings are unaffected by the changing level of heater power being delivered to the wind generator(s). A preferred sensor bridge is thus self zeroing, to provide a zero reading when the oxygen level is zero independently of background gas composition, and is immune to the thermal stresses that would otherwise introduce positional changes and resulting artifacts in sensor of bridge response.